Many electronic systems contain oscillator circuits that generate high-frequency reference signals that are used both internally and externally by the electronic system. For example, the reference signal may be used as a high-speed CPU clock in a data processing device or as a high-frequency local oscillator (LO) carrier signal in a communication device, such as a base station transceiver in a wireless network. The high-frequency reference signal may also be used as a calibration reference in testing and measurement systems.
Generally, it is highly desirable for an oscillator circuit to exhibit low phase noise characteristics. This is usually accomplished by using resonators or tuned circuits that have a high Q-factor. Because quartz crystals and SAW devices exhibit much higher Q-factors than ordinary LC circuits, oscillator circuits that use quartz crystals and SAW devices as frequency determining elements inherently exhibit much lower phase noise than conventional LC oscillator circuits.
Additionally, many electronic systems require that an oscillator circuit be electronically tunable over some required frequency range. It is preferred that the tuning range be comparatively wide and highly linear in the tuning range. Frequently, a varactor diode or other capacitive element is used in conjunction with LC oscillator circuits to provide a tuning capability. The varactor diode interacts with the inductance of the LC oscillator circuit to generate a variable output frequency.
Unfortunately, however, coupling a varactor diode or other capacitive element to a high Q-factor frequency controlling element, such as a SAW device, does not result in a tunable, low phase noise oscillator circuit. As a practical matter, the same electrical characteristics that cause a high Q-factor frequency controlling element to produce good phase noise performance also limit the tuning range of that circuit.
To overcome this limitation, oscillator circuits have been implemented that couple together two or more SAW devices in order to achieve a wider tuning range and low phase noise. However, the use of multiple SAW elements increases the complexity and the cost of these oscillator circuits.
There is therefore a need in the art for improved oscillator circuits that exhibit low phase noise and linear tuning characteristics. In particular, there is a need in the art for a tunable oscillator circuit that uses only a single one-port SAW resonator as a frequency determining element. More particularly, there is a need for a voltage controlled SAW oscillator (VCSO) that exhibits low phase noise and linear tuning across a comparatively wide tuning range (on the order of +/-400 PPM).